Tokyo Electric Power Co. acknowledged on Wednesday a possible leak of highly radioactive water from the earthquake-ravaged Fukishima Daiichi nuclear power plant into the sea.

The operator of the plant said about 300,000 litres of contaminated water has leaked from one of hundreds of steel tanks around the wrecked Fukushima Daiichi plant.

The leak, the fifth since last year involving tanks of the same design, also raised concerns that this could be the beginning of a new disaster — contaminated water leaking from storage tanks one after another.

TEPCO said the leaked water is believed to have mostly seeped into the ground after escaping from the barrier around the tank.

It initially said the leak did not pose an immediate threat to the sea because of its distance — about 500 metres — from the coastline.

The company also said the tank may have been leaking slowly for weeks through a possible flaw on its bottom. That could create extensive soil contamination.

The Japanese nuclear watchdog said on Wednesday it is taking the leakage seriously, and proposed raising the rating to describe it from "an anomaly" to a "serious incident."

"That's what we fear the most. We must remain alert. We should assume that what has happened once could happen again, and prepare for more," watchdog chairman Shunichi Tanaka told a news conference. "We are at a situation where there is no time to waste."

Leak signs overlooked

The watchdog proposed at a weekly meeting to raise the rating of the leak to level three from an earlier level one on an International Nuclear and Radiological event scale of eight. The watchdog, however, plans to consult with the UN nuclear regulatory agency over whether it is appropriate to use the INES evaluation scale on the already wrecked Fukushima plant.

The watchdog urged TEPCO to step up monitoring for leaks and take precautionary measures.

During the meeting, officials also revealed that plant workers apparently have overlooked several signs of leaks, suggesting that their twice-daily patrols were largely just a walk. They have not monitored water levels inside tanks, obviously missed a puddle forming at the bottom of the tank earlier, and kept open a valve on the anti-leakage barrier around the tanks.

Four other tanks of the same design have had similar leaks since last year. The incidents have shaken confidence in the reliability of hundreds of tanks that are crucial for storing water that has been funnelled into the broken reactors to keep melted radioactive fuel cool.

The plant suffered multiple meltdowns following a massive earthquake and tsunami in March 2011 — a level 7 "major accident," the highest on the INES rating and the worst since Chornobyl in 1986.

About 350 of some 1,000 steel tanks built across the plant complex containing nearly 300 million litres of partially treated contaminated water are less-durable ones with rubber seams.

TEPCO says the tanks that have leaked use rubber seams that were intended to last about five years. Ono said TEPCO plans to build additional tanks with welded seams that are more watertight, but will still have to rely on ones with rubber seams.

Cleanup expected to take decades

Figuring out what to do with the radioactive water is among the most pressing issues affecting the cleanup process, which is expected to take decades.

"The growing contaminated water has been one of our biggest concerns since the March 11 accident," said Zengo Aizawa, TEPCO's executive vice president. "The contaminated water remains a problem that could lead to a crisis."

The leaked water's radiation level, measured about 50 centimetres above the puddle, was about 100 millisieverts per hour — the maximum cumulative exposure allowed for plant workers over five years, Ono said.

Contaminated water that TEPCO has been unable to contain continues to enter the Pacific Ocean at a rate of hundreds of tonnes per day. Much of that is ground water that has mixed with untreated radioactive water at the plant.

Workers were pumping out the radioactive puddle on the plant and the remaining water in the tank and will transfer it to other containers. By Tuesday afternoon they had captured only about 4,000 litres, Ono said.